1. Related Applications
This application claims priority to U.S. provisional application Ser. No. 60/672,763, filed Apr. 19, 2005.
2. Field of the Invention
The present invention relates to sectional doors and related safety devices. More particularly, the present invention relates to novel hardware devices designed to improve safety and minimize the risk involved in operating sectional doors that utilize spring mechanisms to facilitate door movement.
3. Background
Large doorways in garages, shops, stores, warehouses and other buildings often use sectional doors to enclose the doorway opening. These doors are generally constructed of wood, vinyl, fiberglass, or metal panels which are joined by hinges and hung from rollers which travel along a fixed track at each side of the door. Sectional doors typically range in size from small storage unit models of just a few feet wide to very large models which accommodate trucks and heavy equipment. Sectional doors are used for residential garages in sizes sufficient to accommodate either one or two vehicles.
The size of sectional doors and the weight of their materials make them relatively heavy and, therefore, difficult to lift without assistance. Many doors also contain insulation and other materials which further add to the door's weight. Even an average-sized residential garage door can weigh several hundred pounds, making it impossible for the average person to lift without assistance.
As a consequence of the weight of sectional doors, mechanisms have been invented to counteract the door's weight, thereby allowing manual operation of the door. The most common method of counteracting a door's weight is accomplished with a counter-spring mechanism using a spring or springs which are displaced elastically as the door is shut, thereby exerting a lifting force on the door as it is closed. This spring force keeps the weight of the door in balance during movement.
Coil springs, in a torsion spring configuration, are often used for these mechanisms. In a torsion spring configuration, the coil spring is deflected or wound around the axis of its helix. In a typical coil spring configuration, as shown in FIGS. 1 and 2, one or more coil springs are wound around a shaft near the top of the door. One end of each coil spring is attached to a mounting bracket which is affixed to the building structure or to the metal frame in which the sectional door is mounted. The other end of the spring is attached to a torsion shaft. A cable drum is likewise mounted on the shaft. A cable is wound around the cable drum. The cable extends to the bottom of the door where it attaches to a bracket. These coil springs are sized and pre-wound or pre-tensioned to ensure that the door remains in balance through the entire path of movement of the door, between closed and open, or open and closed positions.
As the door closes, the cable unwinds from the cable drum thereby twisting the spring and increasing the torsion on the spring and the energy stored within the spring. A properly adjusted spring mechanism will exert a force on a door that is about the same as the weight of the door, allowing a user to open the door with the slightest of lifting effort. This means that the ideal spring mechanism, on an average door, will need to store an amount of energy that is approximately equal to the weight of the door. In terms of force and considering the lever arm of the cable drum, the spring exerts a force of at least twice the weight of the door. Consequently, these spring mechanisms store a great deal of energy that is unleashed as a twisting force. Because of the tremendous forces involved, even well-maintained coil springs will eventually weaken or break. When a spring weakens, the door is no longer in balance. When a spring breaks, it unwinds around its helical axis and releases the stored energy that was balancing the weight of the door.
The coil springs are most likely to break when a door is closed, because that is the point in the traverse of the door when the force stored in the coil spring is greatest—the coil spring is at that point ready to assist in lifting the door. Breakage can occur, however, at any point. This is particularly true in many modern residential and industrial applications where an electric garage door opener is in use. The majority of doors in such situations use more than one coil spring, but the power of an electric garage door opener enables that device to lift the door in many cases when one of the coil springs is weakened or broken, unbeknownst to the user of the door.
When a single remaining coil spring breaks, the only counter-balancing force to the full weight of the door is found in any electric garage door opener that may be attached to the door. These openers are not designed to bear the weight of the door without any assistance from the coil springs. In any case where all the coil springs break, the door will effectively be without a force to counter its full weight. If the coil springs break when the door is fully closed, the door will likely be impossible for an individual to lift without assistance. More troubling, if the coil springs break when the door is not fully closed, the full weight of the door will force it to a closed position, posing a threat of serious injury or even death to any person or animal that lies in its path as it falls. A particular danger may be that of residential homeowners or their children who, unaware that a spring is weakened or broken, release the door's connection to a garage door opener, and then attempt to block the path of a falling door without the benefit of the counterbalancing effect of one or more broken or weakened coil springs.
Inventions in the prior art have used a number of techniques to stop the instantaneous free-fall of a door in a situation where either the coil springs break or are weakened.
In some industrial applications, a hydraulic mechanism is used that restricts the speed of rotation of a cam or drive wheel associated with the door lift mechanism. In these devices, a fluid flows through chambers as the door is raised or lowered. By controlling the size of chambers and the viscosity of the fluid, the amount of force needed to rotate the drive wheel can be changed. Manufacturers select specifications in which the weight of a free-falling door does not provide a sufficient force to rotate the drive wheel at greater than a safe speed, thus controlling the speed of descent for the door. Unfortunately, these hydraulic devices are expensive to manufacture and maintain, and thus inappropriate for many small industrial and residential sectional garage doors.
Solutions used for sectional doors have most often used a mechanical tensioning device to detect a slackening of the tension in a coil spring mechanism. Such a slackening indicates that the coil spring no longer provides a balancing force to the weight of the door. When tension is released in the coil spring, these prior art devices use various techniques to stop the movement of the door.
Although these prior art inventions are effective when a coil spring breaks, they are much less helpful when a coil weakens or is installed incorrectly. A spring that has weakened or that has been incorrectly adjusted or installed generally provides enough tension that a prior art safety device will not detect that a spring is now exerting a much-reduced lifting force on the door. If one or both springs become weakened, the door may drop unexpectedly without triggering a prior art safety device. Such an event might also occur if a user releases a door having a weakened spring from a garage door opener that was preventing the door from falling.
Prior art safety devices pose another potentially serious problem when coil springs break, triggering these devices. Prior art safety devices are typically designed to stop all downward movement of the door, rather than simply the overly rapid descent that poses a danger to users. Because the breakage of a coil spring is most likely to occur when a door is at or near a closed position, the contents of the garage or building are likely to be “locked inside” by these prior art safety devices until a qualified repair technician can arrive on site. Given human nature and the pressures of modern life, an unwary home or business owner is highly likely to attempt to disable or disengage the safety device in order to remove a vehicle, secure a dwelling, or for similar purposes. Individuals who do not understand the mechanisms and forces involved will assume they can manually manipulate the door. Serious injury may result from an attempt to disable or disengage prior art safety devices in order to permit such manual operation.
It is evident, then, that what is needed is a safety device that will prevent the rapid and dangerous descent of a door but not prevent all downward door movement. Such a device would protect against injury by a heavy, falling door. It would also allow a user to disengage the safety device, raise a door with assistance, then carefully lower it to a closed position, or otherwise operate it manually, all the while being protected from grave injury by a safety device that stops a rapid and perilous falling door. Ideally, the invention would allow intuitive use, where a user who has not read an operator's manual can “figure out” how to operate a disabled sectional door manually without risking injury.